The visual contents, whether these are fixed or moving images, are in general creations that benefit from guarantees of exclusivity associated with the creator's rights. Their reproduction is in general permitted only within a strictly defined framework that allows the creators and their beneficiaries to be remunerated.
To ensure that these legal rules are complied with correctly, many systems have been developed to prevent illegal copies or to make the quality of the copies sufficiently degraded to make them unusable.
Within this context, the patent application EP 1 237 369 aims to combat the copying of images by means of a camera while they are being displayed, for example using a camcorder in a movie theatre. In this document, it has been proposed to modulate temporally the amplitude of the brightness of selected pixels representing an anti-piracy message around the value to be displayed at a high rate that makes the message invisible to the human eye but generates artefacts in the sequence filmed by the camcorder. Such a solution requires a modulation at a rate higher than the flicker frequency, which is of around 50 Hz, and therefore applies only to projection systems having a high image refresh rate, at least of around 100 Hz.
In the patent application WO 05/027529, it has been proposed to modulate the colour of the pixels instead of the brightness. This solution requires a modulation at a rate higher than the colour fusion frequency, which is of around 20 Hz as illustrated by FIG. 1.
The color modulation of the selected pixels is described in more detail hereinafter. The processing consists in doubling the frames of the input video, keeping the visual luminance (CIE1931 Y) constant while modifying the two other components (CIE1931 X and Z) in a way invisible to the human eye but affecting the camcorder vision. The two color components X and Z are modulated around the color value to be displayed in a way invisible to the human eye. Every input triplet R0G0B0 describing the color of a pixel in the electric domain leads to the generation of two output triplets R1G1B1 and R2G2B2 which verify the following formulas in the CIE1931 XYZ color space:
                    {                                                                              Y                  1                                =                                                      Y                    2                                    =                                      Y                    0                                                                                                                                                                X                    1                                    +                                      X                    2                                                  =                                  2                  ·                                      X                    0                                                                                                                                                                Z                    1                                    +                                      Z                    2                                                  =                                  2                  ·                                      Z                    0                                                                                                          (        1        )            
Mathematically speaking, the relation (1) means that two modulated signals Xm(t) and Zm(t) are generated based on X0 and Z0 values:
                    {                                                                                                  X                    m                                    ⁡                                      (                    t                    )                                                  =                                                      X                    0                                    ⁡                                      (                                          1                      +                                              cos                        ⁡                                                  (                                                      2                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              m                                                        ⁢                            t                                                    )                                                                                      )                                                                                                                                                                Z                    m                                    ⁡                                      (                    t                    )                                                  =                                                      Z                    0                                    ⁡                                      (                                          1                      +                                              cos                        ⁡                                                  (                                                      2                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              m                                                        ⁢                            t                                                    )                                                                                      )                                                                                                          (        2        )            
where the modulation frequency fm equals 48 Hz for example and where the refresh frequency fr=2fm=96 Hz introduces a time-discretization (∀nεN t=n/fr).
Applying fr, fm and t properties to the equation (2), the following equation is obtained:
                    {                                                                                                  X                    m                                    ⁡                                      (                    t                    )                                                  =                                                      X                    0                                    ⁡                                      (                                          1                      +                                              cos                        ⁡                                                  (                                                      n                            ⁢                                                                                                                  ⁢                            π                                                    )                                                                                      )                                                                                                                                                                Z                    m                                    ⁡                                      (                    t                    )                                                  =                                                      Z                    0                                    ⁡                                      (                                          1                      +                                              cos                        ⁡                                                  (                                                      n                            ⁢                                                                                                                  ⁢                            π                                                    )                                                                                      )                                                                                                          (        3        )            
The equation (3) holds the same properties as the equation (1) over a full period (2π→n={0,1}) with X1=Xm(t)|n=0, X2=Xm(t)|n=1, Z1=Zm(t)|n=0, and Z2=Zm(t)|n=1.
The equation (2) describes an amplitude modulation of the signals X0 and Z0 with a carrier sine wave of frequency fm (48 Hz in our example). Amplitude modulation (AM) is a well-known technique used in analog and digital communications to overcome signal transmission issues by shifting spectrums over high frequencies. In the present case, it allows to generate aliasing artifacts over camcorder acquisitions by increasing the bandwidth of the video signal. The additional constraint Ym(t)=Y0 is provided to ensure invisibility for a human eye, which is able to perceive brightness flicker at 48 Hz while it will not perceive color flicker.
The generation of colour artefacts by use of a camcorder is illustrated by FIGS. 2 to 4. This illustration is made only for the values X0.
The values X0 are supposed to be constant over a short time period. So the original spectrum of the signal X0 can be depicted by FIG. 2. Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier signal changes depending on the amplitude of a modulating signal. A basic AM operation consists in multiplying the modulating signal, for example X0(t) with a carrier signal of frequency fm (in the present example, fm=48 Hz). The spectrum of the modulated signal Xm(t) is shown at FIG. 3. The time discretization (present in both analog and digital cinemas) leads to a periodization of the resulting spectrum with a frequency fr=96 Hz as it can be seen on the upper left and right drawings of FIG. 4.
Since both of the modulation and discretization operations manage to keep extra spectral content out of the 0-48 Hz band (which includes the frequency band in which a human eye would tend to perceive colour flicker), the viewing audience will not notice anything abnormal when watching the modulated movie (lower left drawing on FIG. 4). A camcorder, however, would sample the modulated/discrete spectrum with a sample frequency of either fs=50 Hz (PAL standard: 50 interleaved frames per second) or fs=60 Hz (NTSC standard: 60 interleaved frames per second) which in both cases generate aliasing artefacts since the Nyquist-Shannon sampling theorem is not respected anymore (fs<2fm). Because they appear inside the 0-20 Hz band, these artefacts would then be visible to the human observer, and disturb visualization of the illegally recorded video (lower right drawing on FIG. 4).
However, the actual colour modulation is not sufficient to defeat automatic shutter speed settings of recent camcorders. Indeed, in camcorder technology, the capturing/sampling process of a visual signal first requires its exposure to CCD/CMOS sensors for a certain amount of time. Exposure is also denoted as “shutter speed”, although the shutter used in most video camcorders is not mechanical. Exposure time is the interval during which the CCD/CMOS sensors are exposed to incident light by the shutter, while integration time is defined as the interval during which the clocks of the camcorder are set to trap and retain charge. For a CCD, the integration starts when the CCD is cleared. It is counted from the end of clearing, until the CCD starts to read out. In the frequency domain, shutter could be compared to a low pass filter with exposure time being directly related to the cut-off frequency of the filter.
An issue with the new camcorders is the recent addition of automatic shutter speed adjustment. Auto-shutters measure brightness variations to readapt their own exposure time. As a result, when filming a movie screen, because variations hardly exceed 48 Hz (48 Hz in analog cinema with double-shuttering due to brightness flicker, or 12 Hz in flickerless digital cinema due to motion), the auto-shutter will tend to set exposure time to default values of 1/50 (PAL) or 1/60 (NTSC), thus behaving as a 50/60 Hz low pass filter and removing the colour modulation effect.